100 city pairs might be a way to do the survey, but I suppose that we would find some airlines opening many seats for awards on unpopular routes, when they no longer hope to sell them. If this routes are unpopular, companies shouldn't be given many points for making them available for award. Besides, most of us, I think, are saving our miles for C or F class tickets, or long-haul tickets.

Perhaps we should survey (instead of or as well as your initial idea of 100 US city pairs) the availability of tickets in first or business class between major cities around the world. Even if we lack a way to know the real number of available seats, we could check if there is at least one seat available for a given city pair 331 days before departure, then checking for steadily nearer dates. It would answer the question : how long should I plan a trip in order to redeem successfully my miles. Besides, airline websites do not generally allow to book many seats in one go, so we wouldn't know really how many seats are available, while when a flight has no more available seats, it is no longer possible to book even one seat.